Week 3: Dynamics of Perception & Action
Last week, we saw how Time-to-Contact (\(\tau\)) specifies when an event will happen.
Today, we ask a deeper question:
What can I do?
“The affordances of the environment are what it offers the animal, what it provides or furnishes, either for good or ill.” — J.J. Gibson (1979, p. 127)
Key Concept: Affordances are properties of the environment taken with reference to the animal.
Is “climb-ability” a fact about the stairs? Or a fact about me?
Gibson’s Answer
“An affordance is neither an objective property nor a subjective property; or it is both if you like… It cuts across the dichotomy of subjective-objective.”
It is a Relational Property.
Imagine you are Alice in Wonderland. You drink the potion and shrink to 6 inches tall.
Conclusion: We measure the world in “body-units,” not inches.
Question: When does a step become “unclimbable”?
If you plot the data in Inches: - Tall people stop at 30 inches. - Short people stop at 24 inches. - The curves are different.
If you plot the data in Ratios (\(R/L\)): - Everyone stops at 0.88. - The curves overlap perfectly.
In physics, \(\pi\) refers to a dimensionless ratio (like \(C/D\)). In biology, we have “Intrinsic Metrics.”
\[ \pi = \frac{Riser Height (R)}{Leg Length (L)} \]
How do you know how long your legs are? You don’t look at them.
Hypothesis: Eye Height (\(E\)) specifies Leg Length (\(L\)). If you are standing on 10cm blocks, you feel taller. Does the world look “smaller”?
Reading: Turvey, M.T. & Carello, C. (1995). Dynamic Touch.
Topic: How does a blind person “see” the sidewalk with a cane? The answer lies in the Inertia Tensor.
Dynamics of Perception & Action | Spring 2026